Whilst it is applicable to a wide range of different systems, one use of particular interest is in the context of ADSL transmission technology of the type specified by the ITU-T in G.992.x. This is a DMT (discrete multitone transmission) system in which a large number (of the order of 200) carriers are used, at different frequencies, each carrier being modulated using quadrature amplitude modulation (QAM). The system is adaptive in the sense that the total transmitted data rate is automatically adjusted according to the prevailing properties of the transmission link in use at the time. The setting of the data rate occurs during an initialization procedure in which (inter alia):
(a) the transmitter sends test transmissions to the receiver;
(b) the receiver measures the operating margins;
(c) a decision is made as to the maximum data rate that the link can support;
(d) decisions are made as to the allocation of bits among the different carriers or “bins” according to the relative quality of the link at different frequencies;
(e) these decisions are implemented at both transmitter and receiver.
In most cases, one is considering a transceiver, so that these steps need to be performed for both transmission directions.
In QAM, each transmitted symbol has a phase and amplitude corresponding to a selected one of a limited set of points in the phase plane (referred to as the “constellation”). The operating margin will be discussed in more detail below, but basically is a measure of the extent to which each received symbol is observed to deviate in phase and/or amplitude from the nearest permitted point of the constellation.
In particular it looks at improvement of the resilience of ADSL technology to impairments which vary strongly with time.
We have observed that ADSL is unduly vulnerable to interference from non-stationary noise, such as repetitive electrical impulse noise (REIN) or isolated broadband noise events.
In particular, current ADSL modems have a scalar measure of margin, only suitable for use when the noise impairments do not change significantly with time. In situations where the line is subject to time varying impairments, modems from different vendors behave quite differently, some are essentially unable to calculate the bit rate at which they can operate with stability.
This leads to many modems being unable to stay in synchronization with the modem at the other end, leading to poor customer experience.